The embryonic chick heart exhibits coordinated electrical activity as early as 25 hours of development. Electrical communication among the cells of the embryonic and adult myocardium is fundamental to the establishment of a common rhythm and conduction of the action potential. Low resistance pathways between cells are formed by specialized membrane proteins commonly referred to as gap junctions. It is the purpose of this project to measure the junctional conductance between embryonic heart cells at different stages of development. Applying the whole-cell patch-clamp recording technique to pairs of isolated ventricular cells, the single junctional channel conductance, number of open channels, and dependence of the junctional conductance on voltage gradients across both the cell membrane and the intercellular junction will be determined for each embryonic age studied. Since electrical coupling of heart cells is of fundamental importance to the normal function of the heart, the regulation of the junctional conductance by intracellular ions (H+ and Ca2+) will be investigated under physiological and pathophysiological conditions. Cross-pairing of cardiac tissue types is also proposed as a means to investigate the occurrence of gap junctions between different cardiac cell types.